1. Field of the Invention
The present invention relates generally to ultrasonic transducer design and more particular to the design of ultrasonic transducers for use in vascular catheters to form images of stentonic regions within a blood vessel.
2. Description of the Relevant Art
Intravascular imaging catheters which include ultrasonic transducers are well-known. Most often, imaging catheters capable of providing real time images through 360.degree. have an ultrasonic transducer that is axially mounted, i.e., mounted so that an ultrasonic pulse is transmitted principally along the catheter axis, and use an acoustic mirror to reflect the ultrasonic pulse in a direction perpendicular to the catheter axis. Alternatively, a side looking transducer, i.e., a transducer mounted so that an ultrasonic pulse is transmitted in a direction perpendicular to the catheter axis, may be utilized without a mirror. Examples of several transducer configurations are disclosed in the commonly assigned U.S. Pat. No. 5,000,185 to Yock.
When the ultrasonic transducer is excited it "rings" at a resonant frequency for a small period of time before the ringing is mechanically damped to an insignificant magnitude. Thus, an ultrasonic pulse having a characteristic duration defined in microseconds and length defined in microns is emitted by the transducer. Typically, a transducer includes a transducer element having first and second oppositely disposed major surfaces with a backing element bonded to the second major surface. Most of the ultrasonic energy generated by the transducer element is emitted as pulses propagating in directions perpendicular to the major surfaces. The backing element attenuates reflections of an ultrasonic pulse emitted into the backing element to prevent multiple pulses from being emitted in the direction perpendicular to the first major surface. The primary mechanisms of attenuation are propagation loss, with the amount of attenuation dependent on the distance travelled by the pulse, and reflection loss, with the amount of attenuation determined by mismatch of the impedances at the boundaries of the backing element. This backing element also affects the mechanical damping of the transducer and forms part of the mounting structure of the transducer to the catheter.
To form high-quality intravascular images it is required that only a single ultrasonic pulse is transmitted when the transducer element is excited by an electric pulse and that a single electric pulse be emitted when the transducer element is excited by an ultrasonic pulse. Thus, to prevent the generation of multiple pulses the backing element is fabricated of a sound attenuating material and is thick enough so that reverberations reflecting off the interface of the backing element and a back filling material are attenuated by propagation loss to acceptable levels.
It is generally desirable to minimize the diameter of the catheter to permit its insertion into small diameter blood vessels. The thickness of the backing is usually not critical for an axially mounted transducer because the thickness of the backing does not affect the diameter of the catheter. However, for a side looking transducer often the minimum diameter of the catheter is determined by the thickness of the transducer and backing.
In some applications it is desirable to use a side looking transducer because the mirror strut in the axial system can cause artifacts and the acoustic coupling between the flushing liquid in the catheter and the transducer is improved. However, the use of a side looking transducer having a backing element thick enough to attenuate reverberations to prevent the generation of multiple pulses places a lower limit on the diameter of the catheter which may limit its utility.